This invention is directed to methods of restoring elasticity to elastic material after the elastic material has been stiffened by attaching another material to it.
In order for most nonwoven materials to be stretchable and elastomeric, the individual fibers must be able to move away from one another or even across one another when the material is stretched. Anything that hinders this free movement will cause an increase in the elastic modulus of the material because more force or a higher load is required to stretch the material. Furthermore, if the elastic material is stretched beyond the yield point of the bonds that may exist between the fibers of the elastic material, the material will not contract to the original length. That is, the elastic modulus is not only higher, but there will also be significant non-recoverable stretch (i.e., hysteresis) if the material is stressed beyond its yield point.
For example, when liquid material (e.g., superabsorbent polymer solutions or prepolymer) is applied and/or attached by saturation, coating, printing or spraying onto an elastomeric web and is then polymerized, crosslinked and/or cured, the now solid material bonds groups of fibers together. Bonds between the fibers result in an increase in the elastic modulus of the elastic material because the fibers will not be free to move relative to each other. The bonds also cause the surface of the material to become rough and stiff. Both of these results are undesirable for personal care garments. It is important that material be soft so that personal care garments are comfortable and have an elastic modulus that is low enough to provide the desired ease of stretching for personal care garments.
FIG. 1 shows load vs. elongation extension and retraction curves for an elastomeric high-loft bonded carded web which has been treated with a superabsorbent polymer as well as the same elastomeric high-loft bonded carded web which has not been treated (i.e., untreated) with a superabsorbent polymer. The superabsorbent polymer in this instance was produced using the application and polymerization teachings in U.S. Pat. No. 4,500,315 issued 19 Feb. 1985 to Pieniak et al., and U.S. Pat. No. 6,417,425 issued 09 Jul. 2002 (previously published as PCT Publication No. WO 01/56625) to Whitmore et al., both of which are incorporated herein by reference.
As shown in FIG. 1, the extension curve (i.e., the diagonal line) for the treated (superabsorbent prepolymer material applied, polymerized, crosslinked and dried on) elastomeric high-loft bonded carded web material shown with the upward pointing arrow represents the load (in grams) required to elongate a two (2) inch wide strip of the treated material to the corresponding percentage elongation on the x-axis. The retraction curve (with the downward pointing arrow) represents the load measurements when the treated material was allowed to retract as the deforming load was removed. The decrease in load at all levels of elongation as the material was allowed to retract indicates that the material was weakened from the bonds between the fibers and superabsorbent breaking due to the application of the deforming load.
The sample treated with the superabsorbent polymer exhibited an increase in elastic modulus (in grams). The load required to stretch the material 80% increased from about 100 g/2 inch width of material (for the untreated material) to about 2300 g/2 inch width of material (for the treated material). In addition, when allowed to retract by removing the deforming load significant hysteresis was shown with the treated material. The area between the extension and retraction curves for the treated material is the hysteresis loss.
However, as also shown in FIG. 1, the extension and retraction curves for the untreated elastomeric high-loft bonded carded web with accompanying nearly horizontal pointing arrows represent the load (in grams) required to elongate the untreated material to the corresponding percentage on the x-axis and the load measurements when the material was allowed to retract as the deforming load was removed. As shown, these two lines overlay each other, illustrating that the untreated material maintained its strength and was able to recover to its original unstretched length without any noticeable hysteresis loss.
FIG. 1 illustrates the huge effect on elastic properties that an attached material can have on an elastic material. It is apparent from this data that the elastomeric high-loft bonded carded web will perform significantly differently after the superabsorbent polymer is applied and/or attached. Furthermore, this data illustrates the importance of, and need for, a method of restoring the stiffened elastic material back to its original elastic properties.
There is a need or desire for methods of restoring elasticity to elastic material after the elastic material has been stiffened by applying and/or attaching another material to it.